Initiation of transcription of rDNA in rice

Three direct repeats of 320, 340 and 238 nucleotides were detected upstream to the 5' end of the 18S rRNA gene of an rDNA unit present on a 9.8 kb EcoRI fragment of the rice DNA. The primer extension analysis showed that the site of initiation of transcription is in the 1st repeat at an A, the 623rd nucleotide upstream to the 5' end of the 18S rRNA gene. Different stretches of the intergenic spacer DNA linked to the Chloramphenicol acetyl transferase gene were transcribed in the intact nuclei of rice embryos. The S1 nuclease protection analysis of the transcripts using [32P]-labelled Chloramphenicol acetyl transferase gene as the probe showed the presence of multiple promoters for rDNA transcription.